Panel assembly

ABSTRACT

A panel assembly with a panel and a stringer is disclosed. The stringer has a stringer foot and an upstanding stringer web. The stringer foot has a flange which extends in a widthwise direction between the stringer web and a lateral edge and in a lengthwise direction alongside the stringer web, and a foot run-out which extends between the flange and a tip of the stringer foot. The foot run-out is bonded to the panel at a foot run-out interface. Reinforcement elements, such as tufts, pass through the foot run-out interface into the panel. The reinforcement elements are distributed across the foot run-out interface in a series of rows including an end row nearest to the tip of the stringer foot and further rows spaced progressively further from the tip of the stringer foot. At least the end row has three or more of the reinforcement elements which are distributed along a polygonal curve.

FIELD OF THE INVENTION

The present invention relates to a panel assembly, typically but not exclusively for a composite skin of an aircraft wing.

BACKGROUND OF THE INVENTION

The design of stringer run-outs in composite skins of aircraft wings presents a great technical challenge. High shear and peel stresses can develop locally at the run-out causing the stringer to peel off from the skin. Out of plane stresses develop at the tip of the run-out and since composites are poor in out-of-plane strength, cracks are prone to form at the tip. Additionally, composites are poor in Mode-1 fracture toughness, so these cracks may grow.

A known solution is to clamp the run-out to the skin with a metallic finger plate which is bolted to the stringer foot and skin, as disclosed in US2013/0313391.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a panel assembly comprising: a panel; a stringer comprising a stringer foot and an upstanding stringer web, wherein the stringer foot comprises a flange which extends in a widthwise direction between the stringer web and a lateral edge and in a lengthwise direction alongside the stringer web, and a foot run-out which extends between the flange and a tip of the stringer foot, wherein the foot run-out is bonded to the panel at a foot run-out interface; and reinforcement elements which pass through the foot run-out interface, wherein the reinforcement elements are distributed across the foot run-out interface in a series of rows including an end row nearest to the tip of the stringer foot and further rows spaced progressively further from the tip of the stringer foot, and at least the end row comprises three or more of the reinforcement elements which are distributed along a polygonal curve.

A polygonal curve is a series of line segments connected by corners, where not all of the line segments are co-linear—in other words they do not all lie in a straight line. Distributing the end row along a polygonal curve rather than a straight line enables the reinforcement elements to be more equally loaded by following the expected line of a curved crack front, thus avoiding successive failure of the reinforcement elements.

A further aspect of the invention provides a method of reinforcing a panel assembly, the panel assembly comprising: a panel; a stringer comprising a stringer foot and an upstanding stringer web, wherein the stringer foot comprises a flange which extends in a widthwise direction between the stringer web and a lateral edge and in a lengthwise direction alongside the stringer web, and a foot run-out which extends between the flange and a tip of the stringer foot, wherein the foot run-out contacts the panel at a foot run-out interface. The method comprises: in a design phase, performing a failure test of a test specimen or a failure analysis of a computer model to obtaining a series of rows of points each corresponding with a respective crack profile; and in a reinforcement phase, inserting reinforcement elements through the foot run-out interface in a distribution pattern, and using the series of rows of points from the design phase to determine the distribution pattern of the reinforcement elements across the foot run-out interface.

Optionally the end row comprises four, five, six or more of the reinforcement elements which are distributed along a polygonal curve. Providing a larger number of reinforcement elements in the end row enables the polygonal curve to more closely follow the profile of a tightly curved crack front.

Preferably at least some of the further rows comprise three, four, five, six or more of the reinforcement elements which are distributed along a respective polygonal curve. Providing a larger number of reinforcement elements in the further rows enables each polygonal curve to more closely follow the profile of a tightly curved crack front. Optionally at least some of the further rows have more reinforcement elements than the end row.

Preferably the series of rows do not form a rectilinear grid, or any other grid of identical polygons.

Optionally the polygonal curve, or each respective polygonal curve, has a convex side facing the tip of the stringer foot and a concave side facing away from the tip of the stringer foot.

Optionally at least some of the reinforcement elements are inclined relative to the foot run-out interface. Preferably at least some of the reinforcement elements are inclined at an oblique angle of inclination relative to the foot run-out interface and in a direction of inclination which is either towards or away from the tip of the stringer foot and defines an angle of azimuth relative to the lengthwise direction, and the angle of azimuth lies between −45° and +45°.

Preferably the reinforcement elements are bonded to the foot run-out and/or the panel. This enhances the mechanical performance of the reinforcement elements, prevents leakage problems associated with bolts, and also avoids the structural weakness and lightning strike problems associated with drilled bolt holes.

Preferably each reinforcement element has a diameter less than 1 mm or less than 2 mm.

The reinforcement elements of the end row may be spaced apart along the polygonal curve with a centre-to-centre pitch which is substantially constant, or varies by no more than 10% or 20% from an average centre-to-centre pitch of the end row.

Preferably the reinforcement elements of the end row are spaced apart along the polygonal curve with an average centre-to-centre pitch which is less than 10 mm or less than 5 mm

Preferably the rows are spaced apart with an average row-to-row pitch which is less than 10 mm or less than 5 mm

By way of example, the reinforcement elements may be tufts, Z-pins, or fasteners such as bolts or rivets.

Preferably the foot run-out comprises multiple plies (typically fibre-reinforced composite plies); and the reinforcement elements pass through some or all of the plies of the foot run-out.

Preferably the panel comprises multiple plies (typically fibre-reinforced composite plies); and the reinforcement elements pass through some or all of the plies of the panel.

Preferably the foot run-out and/or the panel are made from a fibre-reinforced composite material.

Preferably the panel has a thickness at the foot run-out interface, and at least some of the reinforcement elements are spaced from the tip of the stringer foot at the point of passing through the foot run-out interface by a distance less than the thickness of the panel at the foot run-out interface.

Preferably at least some of the reinforcement elements are spaced from the tip of the stringer foot at the point of passing through the foot run-out interface by a distance less than 10 mm or less than 5 mm

The stringer web may have the same height along the entire length of the stringer, but more typically it comprises a web run-out which upstands by a height from the stringer foot and terminates at a tip of the stringer web, the height of the web run-out reduces towards the tip of the stringer web, and the foot run-out coincides with the web run-out.

The stringer may have a variety of cross-sectional shapes, including T-shaped, L-shaped, omega (or top-hat) shaped, or J-shaped.

The web may stop short of the foot run-out, so the foot run-out extends further than the web in the lengthwise direction. Alternatively the web may terminate in the same plane as the tip of the foot run-out.

Optionally the foot run-out comprises a first foot run-out flange which extends in the widthwise direction between a first side of the stringer web and a first lateral edge and a second foot run-out flange which extends in the widthwise direction between a second side of the stringer web opposite the first side of the stringer web and a second lateral edge, the reinforcement elements are distributed in first and second series of rows which pass through the first and second foot run-out flanges respectively, each row including an end row nearest the tip of the stringer foot and further rows spaced progressively further from the tip of the stringer foot, the rows of the first series extend laterally away from the first side of the stringer web towards the first lateral edge, the rows of the second series extend laterally away from the second side of the stringer web towards the second lateral edge, the end row of the first series comprises three or more of the reinforcement elements which are distributed along a polygonal curve, and the end row of the second series comprises three or more of the reinforcement elements which are distributed along a polygonal curve.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 is a plan view of a panel assembly;

FIG. 2 is a sectional side view of the panel assembly of FIG. 1;

FIG. 3 is a sectional end view of the panel assembly of FIG. 1 showing multiple stringers;

FIG. 4 is an enlarged plan view of the stringer run-out;

FIG. 5 is an enlarged sectional side of the of the stringer run-out;

FIG. 6 is a schematic view of a tufting head inserting tufts;

FIG. 7 is an enlarged plan view of the stringer run-out;

FIGS. 8-11 are plan views of a test specimen showing crack profiles and data points;

FIG. 12 is a plan view of a stringer run-out with tufts inclined directly away from the tip of the stringer;

FIG. 13 is a plan view of a stringer run-out with tufts inclined directly towards the tip of the stringer;

FIG. 14 is a plan view of a stringer run-out with tufts inclined in alternating directions;

FIG. 15 is a plan view of a stringer run-out with tufts inclined towards the tip of the stringer with an angle of azimuth of +/−45°;

FIG. 16 shows two tufts inclined with an angle of azimuth of 0°;

FIG. 17 shows two tufts inclined with different angles of inclination and different angles of azimuth;

FIG. 18 is a sectional side view of a stringer and panel with no tufts, showing the deformation of the stringer and panel predicted by a finite element analysis (FEA) model;

FIG. 19 is a sectional side view of a stringer and panel with vertical tufts;

FIG. 20 is a sectional side view of a stringer and panel with inclined tufts;

FIGS. 21 and 22 show an aircraft;

FIG. 23 is a sectional side view showing the upper and lower skins of one of the aircraft wings; and

FIG. 24 shows a tuft with non-protruding ends.

DETAILED DESCRIPTION OF EMBODIMENT(S)

A panel assembly shown in FIGS. 1-5 comprises a panel 1 carrying multiple stringers 2. Only one of the stringers is shown in FIG. 1, but three of the stringers are shown side-by-side in FIG. 3. All of the stringers are similar, so only the stringer 2 shown in FIG. 1 will be described in detail.

The stringer 2 has a T-shaped cross-section as shown in FIG. 3, with a stringer foot and an upstanding stringer web 4. The stringer web 4 upstands by a maximum height H from the stringer foot as shown in FIGS. 2 and 3. At the end of the stringer web 4 there is a web run-out 8 which terminates at a tip 7. The web run-out 8 tapers so that the height of the web run-out 8 reduces towards the tip 7 as shown in FIG. 2.

The stringer foot has two symmetrical halves: a flange 3 a and foot run-out part 9 a on one side of the web; and a flange 3 b and foot run-out part 9 b on the other side of the web. The majority of the stringer foot comprises the flanges 3 a,b which each extend in a widthwise direction between the stringer web 4 and a respective lateral edge 5 a,b. Each flange 3 a,b also extends in a lengthwise direction alongside the stringer web 4 up to a respective foot run-out part 9 a,b which coincides with the tapering web run-out 8. The foot run-out parts 9 a,b each extend in the lengthwise direction between a respective flange 3 a,b and a tip 6 of the stringer foot. The tip 6 of the stringer foot is a straight edge running in the widthwise direction perpendicular to the lengthwise direction, although other geometries may be possible. The first foot run-out part 9 a extends in the widthwise direction between a first side 4 a of the stringer web and a first lateral edge, and the second foot run-out part 9 b extends in the widthwise direction between a second side 4 b of the stringer web opposite the first side 4 a of the stringer web and a second lateral edge. In this example the tip 7 of the web and the tip 6 of the stringer foot all lie in the same tip plane 6 a perpendicular to the lengthwise direction.

FIG. 1 shows only one end of the stringer 2. The opposite end of the stringer may be similar, or different to the end of the stringer shown in FIG. 1. The flanges 3 a,b run along the full length of the stringer between the foot run-out parts 9 a,b and the opposite end of the stringer. The stringer foot 3 a, 3 b, 9 a, 9 b is bonded to the panel at a stringer/panel interface (or bondline) which runs the full length of the stringer. The stringer/panel interface comprises a flange interface 10 where the flanges 3 a,b are bonded to the panel, and a foot run-out interface 11 where the foot run-out parts 9 a,b are bonded to the panel.

The panel 1 and the stringer 2 are both made from fibre-reinforced composite materials. More specifically—the panel 1 comprises multiple plies of fibre-reinforced composite material, such as carbon fibres impregnated or infused with an epoxy resin matrix. The stringer 2 is typically made from a similar (or the same) composite material. That is, the stringer foot 3 a, 3 b, 9 a, 9 b and the stringer web 4 are made from multiple plies of fibre-reinforced composite material, such as carbon fibres impregnated or infused with an epoxy resin matrix. Although the stringer foot is illustrated schematically in FIG. 3 as separate from the stringer web 4, in a preferred embodiment the stringer 2 is constructed as two back-to-back L-section pieces—as shown in FIG. 1 of U.S. Pat. No. 5,827,383 for example. FIG. 5 is an enlarged view of the end of the stringer 2 and the panel 1, the horizontal shading indicating the planes of the plies of fibre-reinforced composite material. The panel assembly is manufactured by assembling dry fibre preforms (i.e. carbon fibre without any epoxy resin matrix) then injecting the preforms with epoxy resin matrix material to simultaneously infuse the panel 1 and the stringers 2. This infusion process fully wets the carbon fibre preforms, and the curing of the resin forms bonded joints between the individual plies, and a bonded joint between the stringer 2 and the panel 1 at the stringer/panel interface 10,11.

Reinforcement elements 12, shown in detail in FIG. 5, pass through the full thickness of the stringer foot and the full thickness of the panel 1. In a preferred embodiment each reinforcement element 12 is a tuft—that is, a loop of fibre such as carbon-fibre. The individual tufts 12 may be connected to adjacent tufts by seams, or more typically they are independent with no seams. In other embodiments the reinforcement elements 12 may be Z-pins made of carbon-fibre, steel, copper or any other suitable material. In the following description the reinforcement elements 12 will be referred to as tufts.

FIG. 6 is a schematic view of a tufting head 13 inserting the tufts 12. The tufts are inserted simultaneously, each tuft being inserted by a respective needle inclined at the necessary angle and direction of inclination. An imprint of the required tufting parameters (angle, pattern, density, and profile) is defined in the tufting head 13.

The tufts 12 are inserted before the infusion process, so the infusion process fully wets the tufts, and the curing of the resin forms bonds between the tufts and the resin. Alternatively, the tufts 12 may be inserted after infusion, or the stringers and panel may be laid up as wet prepreg (resin-impregnated carbon fibre).

As shown in FIG. 7, the tufts 12 are distributed across the full extent of the foot run-out parts 9 a,b and also across part of the flanges 3 a,b. As shown in FIG. 5, the tufts pass through the foot run-out interface 11 from the foot run-out parts 9 a,b into the panel 1. A small number of the tufts are also inserted into the flanges 3 a,b—these tufts passing through the flange interface 10 between the flanges and the panel. The rest of each flange is free of tufts as shown in FIG. 2. In other examples, the tufts may only cover part of the foot run-out parts 9 a,b.

The tufts are distributed in first and second series 12 a,b of rows which pass through the first 9 a and second 9 b foot run-out parts respectively. The first series 12 a has twenty-one rows, and the second series 12 b also has twenty-one rows. Each series 12 a,b includes an end row nearest to the tip 6 of the stringer foot and twenty further rows spaced progressively further back from the tip of the stringer foot. As indicated in FIG. 7, the rows of the first series 12 a extend laterally away from the first side 4 a of the stringer web towards the first lateral edge, and the rows of the second series 12 b extend laterally away from the second side 4 b of the stringer web towards the second lateral edge. The end row of the first series 12 a consists of six tufts which are distributed along a polygonal curve 14 a which is not a straight line, and the end row of the second series 12 b consists of six tufts which are distributed along a polygonal curve 14 b which is also not a straight line.

Polygonal curves 14 a,b and 15 a,b are indicated in FIG. 7 for the end rows and one of the further rows, but not for the other rows. Note that the polygonal curves indicated in FIG. 7 are constructed from imaginary straight line segments connecting the tufts. So the straight line segments shown in FIG. 7 do not indicate seams between the tufts (although optionally there may be seams which follow these lines). Note that the corner of each foot run-out part 9 a,b next to the tip 6 is free of tufts.

In this example each row consists of six tufts, but in other embodiments there may be more tufts (for instance sixteen per row) or fewer tufts (for instance three, four or five per row). The centre-to-centre pitch between the adjacent tufts in each row does not vary substantially along each row. In this example the average centre-to-centre pitch (labelled P in FIG. 7) along a row is 3.5 mm and the diameter of each tuft is 0.5 mm The centre-to-centre pitch may vary slightly either side of the average along a row, but preferably by no more than 20% (0.7 mm). The centre-to-centre pitch between rows is also approximately 3.5 mm

In this example, each row has the same number of tufts so the centre-to-centre pitch P does not vary from row-to-row. In another example, the number of tufts per row may increase from row-to-row away from the tip 6, so the centre-to-centre pitch P decreases from row-to-row.

Each polygonal curve has a “C” shape with a convex side facing the tip 6 of the stringer foot and a concave side facing away from the tip 6 of the stringer foot. Each polygonal curve may have a portion where adjacent line segments are co-linear, that is, they lie in a straight line. For instance, the polygonal curve 14 a includes two adjacent line segments which are co-linear. However, none of the polygonal curves are entirely straight.

The distribution pattern for the tufts 12 is determined in a design phase shown in FIGS. 8-11. FIGS. 8 and 9 illustrate a failure test of a test specimen with the same structure as the panel assembly of FIG. 1, except with only a single stringer. The test specimen is stressed by applying a tensile force indicated by arrows, the size of the arrows in FIGS. 8 and 9 indicating the size of the force. The tensile force is applied by holding the panel and stringer at one end, and holding just the panel at the other end. The force is increased in a series of equal steps. FIG. 8 shows the test specimen at the end of a first step, when a crack has formed at the tip 6 of the stringer foot and propagated along the length of the stringer 2 to the position indicated 15 a. A small ultrasonic probe 18 is scanned across the back of the panel 1. An ultrasonic signal is transmitted through the test specimen and echo signals are received. Diminished strength of the echo signal at a particular location defines the presence of a crack (or start of the crack front). The crack locations are marked on the test specimen to obtain a first pair of rows of data points 16a,b shown in FIG. 10 corresponding with the crack profile of FIG. 8. An approximate smooth curve over the data points is drawn to represent the crack front. Then the force is increased so that the crack moves to the position indicated in FIG. 9 at 15 b. The ultrasonic inspection device 18 is scanned again across the foot run-out parts 9 a,b to obtain a second pair of rows of data points 17 a,b shown in FIG. 11 corresponding with the crack profile of FIG. 9. The process is repeated to obtain a full set of data points.

After the design phase of FIGS. 8-11, in a reinforcement phase the tufts 12 are inserted through the foot run-out interface in the distribution pattern of FIG. 7, using the series of rows of data points from the design phase as a guide to determine the distribution pattern.

In another embodiment, during the design phase, a finite element analysis (FEA) is performed on a computer model of the assembly (consisting of the stringer, panel, run-out and tufts) to theoretically predict the crack profile and number of tufts needed in each row to contain the crack growth. This analysis is performed by a suitably programmed computer to obtain the series of rows of data points 16 a,b; 17 a,b each row corresponding with a respective theoretical crack profile.

As shown in FIG. 5, the tufts are inclined at an oblique angle of inclination θ relative to the foot run-out interface 11, in this case about 45° although this angle may vary. Each tuft 12 has a first (upper) portion 121 in the foot run-out 9 a,b and a second (lower) portion 122 in the panel 1. The tufts are inclined in a direction of inclination which is away from the tip 6 of the stringer foot, so that the first portion 121 is further from the tip 6 of the stringer foot than the second portion 122. The direction of inclination of the tufts is also parallel with the lengthwise direction as shown in FIG. 12. Each arrow in FIG. 12 indicates a tuft, with the arrow head showing the direction of inclination. So as shown in FIG. 12 all of the tufts are inclined directly away from the tip 6 of the stringer foot in the lengthwise direction.

FIG. 13 shows an alternative embodiment in which the tufts are inclined in the opposite direction, towards the tip 6 of the stringer foot.

FIG. 14 shows a further alternative embodiment in which columns of tufts are inclined in alternating senses, towards and away from the tip 6 of the stringer foot.

FIG. 15 shows a further alternative embodiment in which the tufts are inclined towards the tip 6 of the stringer foot, but the direction of inclination is not parallel with the lengthwise direction but rather defines an angle of azimuth of +/−45°. In another embodiment, the arrows in FIG. 15 could be reversed so that the tufts are inclined away from the tip 6 of the stringer foot.

FIGS. 16 and 17 illustrate the difference between the angle of inclination and the angle of azimuth for the tufts. FIG. 16 shows a tuft 12 a angled away from the stringer tip as in FIG. 12, and a tuft 12 b angled towards the stringer tip as in FIG. 13. In each case the orthogonal projection of the tuft onto the plane of the foot run-out interface 11 is parallel with the lengthwise direction, so the angle of azimuth relative to the lengthwise direction is zero.

FIG. 17 shows a first tuft 12 c angled away from the stringer tip with an angle of inclination θ1, and a second tuft 12 d angled towards the stringer tip with an angle of inclination θ2. In this case the orthogonal projection of each tuft onto the plane of the foot run-out interface 11 is not parallel with the lengthwise direction. Rather the orthogonal projection of the first tuft 12 c defines an angle of azimuth α1 relative to the lengthwise direction, and the orthogonal projection of the second tuft 12 d defines an angle of azimuth α2 relative to the lengthwise direction. In the case of FIG. 15, half of the tufts have an angle of azimuth α of +45°, and the other half of the tufts have an angle of azimuth α of −45°.

In the case of FIGS. 12 and 13, the angle of azimuth is shown as precisely zero, but in practice manufacturing tolerances will mean that the angle of azimuth of each tuft may deviate from zero by up to 5°, 10° or 15°. However, as long as the angle of azimuth is sufficiently low then the inclined tufts will be sufficiently aligned with the lengthwise direction to provide advantages compared with vertical tufts.

Deformation around the run-out is highly dependent on the geometrical features which lead to formation of the crack and the crack growth. Based on the geometry and the loads, peak tensile and shear stresses are developed at the tip of the run-out or at the crack front after formation of the crack. When vertical tufts are placed behind the crack front (supposing the crack has formed and passed through the tufts) then the tufts reduce the through-thickness tensile stress at the crack tip. However, they do not significantly affect the transverse shear stress. Inclining the tufts behind the crack front considerably reduces the peak tensile and the shear stresses at the crack tip.

FIG. 18 is a view of a stringer 2 and panel 1 with no tufts, showing the deformation of the stringer and panel predicted by a finite element analysis (FEA) model after a crack has propagated between them. Note that there is a distinct upward kink at the tip of the stringer 2. FIG. 19 shows the output of the same FEA model but with vertical tufts 123. Note that the kink is still present. FIG. 20 shows the output of the same FEA model but with tufts 124 inclined away from the tip of the stringer. Note that there is no kink in the stringer run-out. This relative lack of deformation of the stringer run-out is beneficial because it reduces tensile and shear stresses at the crack front.

The inclined tufts modify the local load path as shown in FIG. 5—the arrows indicating the load path from the panel 1 to the stringer 2 via the inclined tufts. This results in reduced load flow in a local zone 20 at the tip of the stringer minimising the chance of formation of a crack in this local zone. Calculations show that for a tufting density of 4%, the shear stiffness of the joint is doubled for tufts inclined at 45° in the region that is still bonded, which will alleviate the transverse shear stresses at the crack front or at the tip of the run-out before a crack is formed, thereby reducing the chance of a crack spreading by shear, or even forming in the first place. Beneficially, the first row of inclined tufts is sufficiently close to the tip 6 that they extend through the plane 6a of the tip as shown in FIG. 5.

The first row of tufts is positioned as close as possible to the tip 6 of the stringer foot, in order to provide this reduced load flow in the local zone 20. In the case of FIG. 5, the distance D between the first row of tufts and the tip 6 of the foot run-out, at the point where the tuft passes through the interface 11, is less than the thickness of the panel 1 at the interface 11. The thickness of the panel 1 at the interface 11 could vary—for example it might be between 3 mm and 10 mm Typically the distance D is greater than 3.5 mm but less than 10 mm

FIGS. 21 and 22 show an aircraft with a pair of wings. Each wing has an upper (low pressure) skin 30 and a lower (high pressure) skin 31. FIG. 23 is a schematic sectional view through one of the aircraft wings showing only the upper and lower skins 30, 31. Each skin comprises a panel assembly as shown in FIGS. 1-5, so it will not be described again in detail. The wing is attached to the fuselage at its root, and extends in a spanwise direction to its tip. The stringers extend in the spanwise direction from an inboard end at the root of the wing to an outboard end towards the tip of the wing. FIG. 23 shows only the outboard end of the stringer 2. The inboard end of the stringer typically has a different construction, running into the wing root joint and being held down by cleats for example.

The aerodynamic loads acting on the wing cause it to bend upwards so the lower skin is in tension. Therefore in the lower skin 31 the tufts are inclined in a direction of inclination which is away from the tip of the stringer foot. So the first (upper) portion 121 of each tuft in the stringer foot is further from the tip 6 of the stringer foot than the second (lower) portion 122 of the tuft in the lower skin 31.

The upward bending of the wing causes the upper skin 30 to be in compression, so the direction of inclination of the tufts is reversed compared with the lower skin. So in the upper skin the tufts are inclined in a direction of inclination which is towards the tip 6 of the stringer foot, so that the first (lower) portion 121 of each tuft in the stringer foot is closer to the tip 6 of the stringer foot than the second (upper) portion 122 of the tuft in the upper skin 30.

The ends of the tufts in FIGS. 1-23 are shown protruding from the composite structure. In some applications this may be acceptable, but in the case of an aircraft wing as in FIG. 23, ideally the end of each tuft is bent so as to be flush with the surface, as shown by the single tuft 19 in FIG. 24. This is particularly important on the outer aerodynamic surface of the wing skin.

Where the word ‘or’ appears this is to be construed to mean ‘and/or’ such that items referred to are not necessarily mutually exclusive and may be used in any appropriate combination.

Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims. 

1. A panel assembly comprising: a panel; a stringer comprising a stringer foot and an upstanding stringer web, wherein the stringer foot comprises a flange which extends in a widthwise direction between the stringer web and a lateral edge and in a lengthwise direction alongside the stringer web, and a foot run-out which extends between the flange and a tip of the stringer foot, wherein the foot run-out is bonded to the panel at a foot run-out interface; and reinforcement elements which pass through the foot run-out interface, wherein the reinforcement elements are distributed across the foot run-out interface in a series of rows including an end row nearest to the tip of the stringer foot and further rows spaced progressively further from the tip of the stringer foot, and at least the end row comprises three or more of the reinforcement elements which are distributed along a polygonal curve.
 2. The panel assembly of claim 1 wherein the reinforcement elements are bonded to the foot run-out and/or the panel.
 3. The panel assembly of claim 1 wherein at least the end row comprises four, five, six or more of the reinforcement elements which are distributed along the polygonal curve.
 4. The panel assembly of claim 1 wherein at least some of the further rows comprise three or more of the reinforcement elements which are distributed along a respective polygonal curve.
 5. The panel assembly of claim 1 wherein at least some of the further rows comprise four, five, six or more of the reinforcement elements which are distributed along a respective polygonal curve.
 6. The panel assembly of claim 1 wherein the series of rows do not form a rectilinear grid.
 7. The panel assembly of claim 1 wherein the series of rows do not form a grid of identical polygons.
 8. The panel assembly of claim 1 wherein the reinforcement elements of the end row are spaced apart along the polygonal curve with a centre-to-centre pitch which is substantially constant, or varies by no more than 10% or 20% from an average centre-to-centre pitch of the end row.
 9. The panel assembly of claim 1 wherein the reinforcement elements of the end row are spaced apart along the polygonal curve with an average centre-to-centre pitch which is less than 10 mm.
 10. The panel assembly of claim 1 wherein the polygonal curve, or each respective polygonal curve, has a convex side facing the tip of the stringer foot and a concave side facing away from the tip of the stringer foot.
 11. The panel assembly of claim 1 wherein the reinforcement elements are tufts or Z-pins.
 12. The panel assembly of claim 1 wherein each of the reinforcement elements has a diameter less than 2 mm.
 13. The panel assembly of claim 1 wherein the foot run-out comprises multiple plies; and the reinforcement elements pass through some or all of the plies of the foot run-out.
 14. The panel assembly of claim 1 wherein the panel comprises multiple plies; and the reinforcement elements pass through some or all of the plies of the panel.
 15. The panel assembly of claim 1 wherein the panel has a thickness at the foot run-out interface, and at least some of the reinforcement elements are spaced from the tip of the stringer foot at the point of passing through the foot run-out interface by a distance less than the thickness of the panel at the foot run-out interface.
 16. The panel assembly of claim 1 wherein the stringer web comprises a web run-out which upstands by a height from the stringer foot and terminates at a tip of the stringer web, the height of the web run-out reduces towards the tip of the stringer web, and the foot run-out coincides with the web run-out.
 17. The panel assembly of claim 1 wherein the foot run-out comprises a first foot run-out flange which extends in the widthwise direction between a first side of the stringer web and a first lateral edge, and a second foot run-out flange which extends in the widthwise direction between a second side of the stringer web opposite the first side of the stringer web and a second lateral edge, the reinforcement elements are distributed in first and second series of rows which pass through the first and second foot run-out flanges respectively, each row including an end row nearest the tip of the stringer foot and further rows spaced progressively further from the tip of the stringer foot, the rows of the first series extend laterally away from the first side of the stringer web towards the first lateral edge, the rows of the second series extend laterally away from the second side of the stringer web towards the second lateral edge, the end row of the first series comprises three or more of the reinforcement elements which are distributed along a first polygonal curve, and the end row of the second series comprises three or more of the reinforcement elements which are distributed along a second polygonal curve.
 18. The panel assembly of claim 1 wherein at least some of the reinforcement elements are inclined relative to the foot run-out interface.
 19. A method of reinforcing a panel assembly, the panel assembly comprising: a panel; a stringer comprising a stringer foot and an upstanding stringer web, wherein the stringer foot comprises a flange which extends in a widthwise direction between the stringer web and a lateral edge and in a lengthwise direction alongside the stringer web, and a foot run-out which extends between the flange and a tip of the stringer foot, wherein the foot run-out contacts the panel at a foot run-out interface, the method comprising: in a design phase, performing a failure test of a test specimen or a failure analysis of a computer model to obtaining a series of rows of points each corresponding with a respective crack profile; and in a reinforcement phase, inserting reinforcement elements through the foot run-out interface in a distribution pattern, and using the series of rows of points from the design phase to determine the distribution pattern of the reinforcement elements across the foot run-out interface.
 20. An aircraft comprising a panel assembly according to claim
 1. 